This application is based on and claims priority from Danish application no. PA 2000 01475 filed Oct. 4, 2000, and European Patent Application No. 01610048.9 filed May 10, 2001, the disclosures of which are incorporated by reference herein.
The present invention relates to a hearing aid with a directional characteristic, comprising at least two spaced apart input transducers.
Hearing aids comprising two input transducers and having a directional characteristic are well known in the art. A sound wave that impinges on a hearing aid of this type at a specific angle is received by the two input transducers with an arrival time difference defined by the distance between the input transducers, the velocity of sound, and the impinging angle. The output signals of the two input transducers are combined to form the directional characteristic of the hearing aid. When the output signal of the input transducer receiving the sound wave first is delayed by an amount that is equal to the arrival time difference of the corresponding sound wave and subtracted from the output signal of the other input transducer, the two output signals will cancel each other. Thus, a notch is created in the directional characteristic of the hearing aid at the receiving angle in question. By adjusting the delay of the input transducer signal before subtraction, the angular position of the notch in the directional characteristic may be adjusted correspondingly.
It is also well known that the frequency response of subtracted signals originating from a sound source in the surroundings of the hearing aid, i.e. the transducer signals are correlated signals, has a 6 dB/octave positive slope. Thus, low frequencies are attenuated for correlated signals while this is not the case for non-correlated signals, i.e. neither transducer noise nor wind noise is attenuated. Therefore, the signal to noise ratio is reduced in a prior art directional hearing aid compared to an omnidirectional hearing aid.
Notch formation requires that the two input transducers are identical, i.e. they have identical parameters, such as sensitivities and phase responses. Typically, identically manufactured input transducers exhibit sensitivity differences of the order of 6 dB and phase differences of the order of 10xc2x0. Directional characteristics can not be formed with input transducers with phase and sensitivity differences of this magnitude. Selection of paired input transducers may reduce the sensitivity differences to 0.5 dB and phase differences to 2xc2x0 which may still not lead to notch formation in the directional characteristic. Further, aging may increase these differences over time.
In U.S. Pat. No. 6,272,229, a hearing aid with adaptive matching of input transducers is disclosed. According to the disclosure, differences in sensitivity and phase response are compensated utilizing specific circuitry continuously determining the differences and compensating for them. The differences are determined based on the sound signals received by the input transducers. No additional signals are needed. Selection of input transducers is eliminated and differences between circuitry processing each of the respective input transducer signals and differences created by aging or other influences are automatically compensated.
In a hearing aid with a plurality of input transducers, the output signals from the respective input transducers may not be generated from the same sound source. For example, when the hearing aid is operated in a silent environment, each of the input transducer signals contains only noise generated by the respective input transducer itself. Thus, in this case, the output signals are generated by independent and thus, non-correlated signal sources, namely the individual input transducers. Likewise, signals generated by the two input transducers in response to wind, i.e. wind noise, are not correlated since air flow at the hearing aid is turbulent. Thus, also in this case, the output signals are generated by independent signal sources. Further, the input transducer signals are clipped at high input levels by the A/D converters converting the input transducer signals to digital signals. Typically, signals are clipped at different signal levels because of different input transducer sensitivities and, thus, clipped signals may also be non-correlated and appear to have been generated by independent signal sources.
When the input transducer signals are generated by independent signal sources, the above-mentioned prior art input transducer matching technique falls apart since, typically, the determined phase and sensitivity differences will be dominated by differences in the generated signals and will not be related to differences in input transducer parameters.
It is an object of the present invention to provide a hearing aid with a directional characteristic that overcomes the above-mentioned disadvantages of the prior art.
This object is fulfilled by a hearing aid with a directional characteristic wherein transducer signal type, such as transducer noise, wind noise, sound emitted from a sound source located in the surroundings of the hearing aid, distorted signals, such as clipped signals, slew rate limited signals, etc, etc, is determined, and wherein signal processing in the hearing aid, such as transducer matching, filtering, signal combination, etc, is adapted according to the determined signal type. For example, the directional characteristic may be switched to an omnidirectional characteristic when at least one of the input transducer signals is dominated by noise or distortion, and/or adaptive matching of input transducers may be put on hold while at least one of the input transducer signals is dominated by noise or distortion.
Thus, the above-mentioned and other objects are fulfilled by a hearing aid comprising a first and a second input transducer for transforming an acoustic input signal into respective first and second input transducer signals, a first signal processor having a first input that is connected to the first input transducer signal and a second input that is connected to the second input transducer signal for generation of a third electrical signal by processing and combining the input signals, an output transducer for transforming the third electrical signal into an acoustic output signal, a correlation detector for detection of non-correlated first and second processor input signals and for generation of one or more control signals including a first control signal in response to the detection so that transducer signal processing can be adapted according to the detection.
The hearing aid may further comprise an adaptive matching circuit with first and second inputs that are connected with the respective first and second input transducer signals and first and second outputs that are connected to the respective first and second processor inputs for modification of amplitude and phase responses of the first and second output signals in response to determinations of difference in the amplitude and phase responses so that the resulting amplitude and phase responses of the first and second output signals are adjusted to be substantially identical, and wherein the correlation detector generates a second control signal that is connected to the adaptive matching circuit for inhibition of adaptive matching upon detection of non-correlated signals.
The first and second control signals may be identical signals.
In a hearing aid according to this embodiment of the present invention, transducer differences, such as differences in sensitivities, phase responses, etc, are continuously determined when the transducer signals are correlated, e.g. when the transducer signals are generated in response to a sound source located in the surroundings of the hearing aid so that in this case the hearing aid continuously adapts to changes in transducer parameters. When non-correlated signals are detected, e.g. when the transducer signals are dominated by non-correlated signals, such as when at least one transducer signal is dominated by, e.g. transducer noise, wind noise, signal clipping, etc, updating of determined values of differences in transducer parameters is not performed rather, for example, the transducer parameter compensating circuitry remains set according to the latest updated values of the differences.
The correlation detector may comprise one or more signal level detectors for detection of respective input transducer signal levels. For example, the first and the second control outputs may be set to a logic xe2x80x9c1xe2x80x9d when the detected signal level is greater than a predetermined threshold level such as 2 dB below the saturation level of the A/D converters for converting the input transducer signals to digital signals. The first and the second control outputs may be reset to a logic xe2x80x9c0xe2x80x9d when the detected signal levels return to values below the predetermined thresholds. The level detector may further have hysteresis so that the control outputs may be set when the detected signal level is above a first predetermined threshold level and reset when the detected signal level returns to a value below a second predetermined threshold level that is lower than the first threshold level.
The signal level may be an amplitude level, a root mean square level, a power level, etc, or the ratio between such levels and a corresponding reference quantity, e.g. in dB. Further, the level may be determined within a specific frequency range.
The signal level detectors may further comprise slew rate detectors for detection of rapid signal changes since slew rate limitations of circuitry that processes input transducer signals may distort these signals. The signal level detector may for example comprise a slew rate threshold so that the first control output is set e.g. to logic xe2x80x9c1xe2x80x9d if an increase in absolute value of the difference between one sample and the next is greater than or equal to the slew rate threshold.
Typically, wind noise generates transducer signals at very high levels even at low wind speeds thus, wind noise will typically be detected utilizing a signal level detector as described above.
The hearing aid may further comprise a frequency analyzer for determination of the frequency content of input transducer signals, e.g. for discrimination between signal type. For example, wind noise and clipped signals may be distinguished based on their frequency content, and signal processing may be adapted accordingly.
Further, the signal level detectors may be used for detection of the level of a noise signal whereby wind noise may be distinguished from transducer noise since, typically, transducer noise is a low level signal while wind noise is a high level signal.
Thus, according to the present invention, at least three types of signals may be identified, i.e. transducer noise signals, wind noise signals, and signals from sound sources located in the surroundings of the hearing aid. Further, distorted signal types, such as clipped signals, slew rate limited signals, etc, may be identified.
As already mentioned, transducer signals dominated by transducer noise, wind noise, and/or signal distortion are not correlated since the signal sources are substantially independent of each other. The opposite is true for transducer signals generated in response to a specific sound source located in the surroundings of the hearing aid. Such signals differ only by the arrival time difference caused by the distance between the transducers and by differences caused by transducer differences, i.e. such signals are highly correlated. Thus, signals of this type may be distinguished by calculation of cross-correlation values of input transducer signals.
According to an embodiment of the present invention, the correlation detector comprises a second signal processor that is adapted to calculate a cross-correlation value of signals derived from the transducer signals. Output transducer signals with a cross-correlation value within a predetermined range of cross-correlation values are treated as correlated signals.
For example, a cross-correlation value r0 may be calculated as an approximation to or an estimate of a value r defined by the following equation:   r  =                    ∑        XY            -                        ∑                      X            ⁢                          ∑              Y                                      N                                      (                                    ∑                              X                2                                      -                                                            (                                      ∑                    X                                    )                                2                            N                                )                ⁢                  (                                    ∑                              Y                2                                      -                                                            (                                      ∑                    Y                                    )                                2                            N                                )                    
wherein X is a sampled signal derived from the first signal, Y is a sampled signal derived from the second signal, and N is the number of samples.
It is noted that r ranges from xe2x88x921 to 1 and that r=1 for identical signals X and Y and r=1 for inverted signals X and Y and r=0 for signals with no mutual correlation.
It is also noted that the equation is simplified for signals having DC-values equal to zero, i.e. xcexa3X=0 and xcexa3Y=0 in the equation.
In a preferred embodiment of the present invention, the correlation value r0 is calculated from a particularly simple approximation to the equation wherein the signals X and Y are digitized in one bit words, i.e. the sign of the signals X and Y are inserted in the equation.
It is even more preferred to calculate the correlation value r0 as a running mean value wherein a predetermined value xcex941 is added to the sum when sign(X)=sign (Y) and wherein a predetermined value xcex942 is added to the sum when sign(X)xe2x89xa0sign (Y). If, for example, xcex941=1, and xcex942=0, r increases towards the value 1 when X and Y have identical signs, and r decreases towards xc2xd when X and Y have opposite signs. Since non-correlated signals, such as transducer noise or wind noise, change sign independently of each other and thus, will have identical signs half the time while signals generated in response to a specific sound source are highly correlated and have the same sign substantially all the time.
In an embodiment of the invention, the first signal processor is adapted to process the first and second electrical signals for formation of an omnidirectional characteristic upon detection of non-correlated signals, e.g. by signal level detection, by cross-correlation calculation, etc. The omnidirectional characteristic may be formed by selecting the first or the second electrical signal as the third electrical signal whereby signal to noise ration is improved compared to a directional characteristic, or, the omnidirectional characteristic may be formed by averaging the first and second electrical signals whereby signal to noise ratio may be further improved and clipping or slew rate distortion reduced if, for example, only one of the signals is clipped or slew rate limited.